replacing XAtoms with StructureAtoms
changing location of module imports to avoid cyclic ImportErrors
Moving coordinate attributes/methods from Atom(s) to XAtom(s).

Moved code pertaining to atom coordinates from the
Atom and Atoms classes to the XAtom and XAtoms classes,
respectively.
removing NeighborAtoms module
updating module imports
replacing NeighborAtoms with StructureAtoms class
fixing test module typo
removing NeighborAtoms import
updating docstrings
setting python intersphinx_mapping to python3 docs
updating MANIFEST includes/excludes
updating setup.py script to use numpy.distutils.core setup function
adding tests directory to core.physics package
adding/updating numpy.distutils.core setup modules
tweak setup script for `develop` option
adding `--version` action to argparser